fix docs for displayBrightness and colorDepth return on error values.
fix docs for displayBrightness and colorDepth return on error values.

Task-number: QTMOBILITY-420

qtproject/qt-mobility,tmcguire/qt-mobility,enthought/qt-mobility,qtproject/qt-mobility,kaltsi/qt-mobility,kaltsi/qt-mobility,tmcguire/qt-mobility,qtproject/qt-mobility,enthought/qt-mobility,KDE/android-qt-mobility,KDE/android-qt-mobility,qtproject/qt-mobility,enthought/qt-mobility,tmcguire/qt-mobility,KDE/android-qt-mobility,enthought/qt-mobility,qtproject/qt-mobility,tmcguire/qt-mobility,KDE/android-qt-mobility,qtproject/qt-mobility,kaltsi/qt-mobility,kaltsi/qt-mobility,kaltsi/qt-mobility,kaltsi/qt-mobility,enthought/qt-mobility,enthought/qt-mobility,tmcguire/qt-mobility